Electron discharge apparatus



June 2, 1959 Inventor D. C. ROGERS A tforne y Sites ELECTRON DISCHARGE APPARATUS Douglas Cecil Rogers, London, England, assignor to International Standard Electric Corporation, New York, N.Y., a corporation of Delaware The present invention relates to electron discharge apparatus employing electron beams.

In electron discharge apparatus it is impossible to produce a complete vacuum: some residual gas will always be present within the hermetic enclosure of the discharge device. Furthermore, due to various causes such as electron bombardment of the electrodes, there is a tendency throughout the life of a discharge tube for gas to be liberated from the inner surfaces of the tube envelope and from the electrodes. The presence of this residual gas, particularly in the case of travelling wave amplifiers, has a considerable effect upon the performance of the discharge device. The measurement of the residual .gas pressure during operation, and particularly during factory tests on these discharge tubes, is highly desirable.

In an electron beam tube positive ions tend to be ac celerated along the electron beam path in the reverse direction to the beam propagation.

In accordance with the present invention there is provided the method of testing an electron discharge device comprising collecting and measuring the positive ion current flowing towards the cathode during operation of the said device.

Such a method of testing has hitherto been impossible because of the lack of a suitable electrode to which ion current could be measured. It appears probable that the life of a travelling wave tube, for example, is intimately connected with the residual gas pressure and, therefore, facility for measuring ion current during use of the tube, as well as during manufacture is advantageous. Now the positive ions may bombard the cathode surface of a tube as normally constructed with resultant damage to the surface and evolution of further gas. To avoid this trouble it has previously been proposed, in an electron gun, to form a central aperture in the cathode emitting surface and to place behind this aperture a small cup holding getter material to absorb the ions.

From the point of view of the construction of an electron dischange device the present invention provides an electron discharge device comprising an electron gun for producing an electron beam and having an indirectly heated cathode, the emissive portion of which contains a central aperture, and an electrode insulated from the cathode situated behind the cathode aperture to collect positive ions accelerated along the electron beam path through the said central aperture.

With such a device the residual gas pressure can be measured in terms of the positive ion current and from a further aspect, the invention provides the method of measurement of the residual gas pressure in an electron discharge device comprising the measurement of the positive ion current flowing through a central aperture in the cathode of the said device in the opposite direction to the normal electron current to an ion collector electrode disposed behind the said central aperture.

An embodiment of the invention will be described with reference to the accompanying drawing, which illustrates diagrammatically, on a very much enlarged scale, the essential components of an electron gun and indicates the direct current connections between the gun electrodes.

In the drawing an indirectly heated cathode 1 is formed by a tubular metallic member 2 mounted upon an insulator 3 which is clamped between ceramic rings 4 and 5, which are shown partly cut away. The emissive surface of the cathode is provided by a spherically shaped end cap 6 fitting into the end of the tube 2. The end cap 6 is coated with electron emissive material, and contains a central aperture 7. A heater winding 8 is shown supported inside the end of the tube 2, adjacent the cap 6. A focussing electrode 9 and accelerating anode 10, which is apertured at 11, are indicated fragmentarily. A metallic rod 12. is mounted behind the aperture 7 within the tube 2 by means of an eyelet 13, secured to the insulator 3 so as to be insulated from the cathode.

In operation the tocussing electrode 9 is maintained at cathode potential by means such as the connection 14 and the anode 10 is maintained at a positive potential with respect to the cathode by means of a source of DC. potential indicated by the battery 15. A convergent beam of electrons is drawn towards the anode 10 and passes through the aperture 11, to be utilised in the remainder of tube which is not shown.

The collector electrode 12 is preferably maintained at a negative potential with respect to the cathode by means such as a source indicated by the battery 16. Positive ions flowing towards the cathode will tend to be focussed through the aperture 7 and be collected by the collector electrode 12. The resulting ion current may then be measured in a suitable manner such as by means of a microammeter indicated at 17. The negative polarising potential of electrode 12 is not essential; the source 16 may be omitted, if desired, at the expense of somewhat less satisfactory operation.

By means of the construction of electron gun and the circuit arrangement of the present invention, the positive ion current generated in an electron beam tube can be continuously measured both during factory processing and testing, and also, if desired, during subsequent operation of the discharge device.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What I claim is:

1. An electron discharge device comprising an electron gun for producing an electron beam and having an indirectly heated cathode, the emissive portion of which contains a central aperture, an accelerating electrode for accelerating electrons emitted from said cathode, and an electrode insulated from the cathode situated behind the cathode aperture to collect positive ions accelerated along the electron beam path through the said central aperture and means for biasing said electrode negative with respect to said cathode.

2. An electron gun comprising an indirectly heated cathode, a focussing electrode, an anode and an ion collector electrode in which the said cathode comprises a tubular metallic member mounted upon an insulator, a

centrally iapertured' efxd m coated Withelectton emissive material closing one end of the said tubu1ar memher, a cathode heater mounted within the said tubular member adjacent the said end cap, and an ion collector electrode mounted on the said insulator behind the central aperture in the said end cap.

"References Cited 'in thefiie "of this patent UNITED -S'I-AI ES PATENTS Anderson Ian. 19, 1943 Varian July 27, 1948 Dorgelo Apr. 3, 1951 Lander Jan. 13, 1953 Clark et a1. May 19, 1953 FOREIGN PATENTS Great Britain May 19, 1954 

